1. Field of the Invention
This invention relates to loudspeakers, and in particular, to a diaphragm for a loudspeaker that significantly improves the quality of sound and the usable life of the loudspeaker.
2. Related Art
A typical loudspeaker, as shown in FIG. 1, has a cone and/or dome diaphragm that is driven by a voice coil that is immersed in a strong magnetic field. The voice coil is electrically connected to an amplifier and, when in operation, the voice coil moves back and forth in response to the electromagnetic forces on the coil caused by the current in the coil, generated by the amplifier, and the stationary magnetic field. The cone and voice coil assembly is typically suspended by a “spider” and a “surround,” a flexible connector frame. This suspension system allows the cone and coil assembly to move as a finite excursion piston over a limited frequency range. Like all mechanical structures, cones and domes have natural modes or “mode peaks” commonly called “cone break-up.” The frequency at which these modes occur is largely determined by the stiffness, density, and dimensions of the diaphragm, and the amplitude of these modes is largely determined by internal damping of the diaphragm material. These mode peaks are a significant source of audible coloration and, as a result, degrade the performance of the loudspeaker system.
Designers have tended to take two paths to solve the cone break-up problem. For small diaphragms such as those found in dome tweeters, aluminum and titanium are commonly used. These titanium and aluminum diaphragms typically feature a thin anodized layer to provide a specific color to the visible surface, or to protect the metal from sunlight, humidity, or moisture. In contrast, for larger diaphragms, such as those found in subwoofers, softer materials such as polymers or papers are commonly used.
When using metal diaphragms, the dome dimensions can be manipulated such that the first natural modes of the dome are above the frequency range of human hearing. FIG. 2 shows the frequency response of a typical 1″ titanium dome tweeter (note the large mode peak 22 at 25 kHz). The amplitude of these modes is usually very high because metals have very little internal damping. For diaphragms larger than approximately 1″, the dome modes fall into the audible range. These modes are plainly audible as coloration because of the high amplitude of the modes. FIG. 3 shows the frequency response of a typical 3″ titanium dome mid-range speaker (note several large peaks 24, 26, and 28 at 11 kHz, 16 kHz, and 18 kHz). Since the modes fall into the audible range as the size of the diaphragms increase, metal diaphragms become less desirable for larger diaphragms.
For larger diaphragms, softer materials such as polymers or papers are commonly used. These materials have several natural modes in the band in which they operate. However, the internal damping of these materials is high enough so that most of these modes do not cause audible coloration. The remaining modes are either compensated for in other parts of the loudspeaker system design, resulting in increased costs, or are not addressed at all, resulting in lower performance. FIG. 4 shows the frequency response of a typical 5″ woofer with a polypropylene cone (note the large mode peaks 30 and 32 at 4 kHz and 5 kHz).
As an alternative to metal, paper and polymers, ceramic materials such as alumina or magnesia may be used. These ceramic materials offer significantly higher stiffness numbers and slightly better internal losses than typical metals such as titanium or aluminum. As a result, the natural modes of diaphragms made of these materials are moved higher in frequency and reduced in amplitude and, thus, reduce audible coloration. Unfortunately, pure ceramics are very brittle and are prone to shattering when used as loudspeaker diaphragms. Additionally, making diaphragms of appropriate dimensions can be very expensive. As a result, pure ceramic loudspeaker diaphragms have not become common.
Table I shows the structural parameters for several common diaphragm materials.
TABLE IPROPERTIES OF DIAPHRAGM MATERIALSYoung'sModulusSpeed ofInternal LossMaterial(Stiffness)DensitySound(damping)Paper 4 × 109 Pa0.4 g/cm31000 m/sec0.06 Polypropylene 1.5 × 109 Pa0.9 g/cm31300 m/sec0.08 Titanium110 × 109 Pa4.5 g/cm34900 m/sec0.0003Aluminum 70 × 109 Pa2.7 g/cm35100 m/sec0.0003Alumina340 × 109 Pa3.8 g/cm39400 m/sec0.004 
As yet another alternative to metal, paper or ceramic diaphragms, some designers have designed diaphragms that are made of both ceramic and metal. These diaphragms are formed by applying a skin of alumina or ceramic on each side of the aluminum core or substrate. The alumina thus supplies the strength and the aluminum substrate supplies the resistance to shattering. It has high internal frequency losses. The resulting composite material is less dense and less brittle than traditional ceramics, yet is significantly stiffer, and has better damping than titanium. It also resists moisture and sunlight better than any polymer and is at least as good as other metals for providing such resistance.
These ceramic/metal cones are typically 3 mils. thick with a 2.6 mils. thick substrate of aluminum and 0.2 mil. thick layers of alumina one on each side of the substrate. In these prior art ceramic/metal cones, the metal substrate represented approximately 87% of the total thickness of the cone. Because of the prior art methods of manufacturing the cones, the amount of ceramic that could be applied to the metal substrate was limited to a depth of about 1/10 of a mil and therefore the quality that could be achieved through this method was similarly limited. Thus, a need exists for a method of anodizing the metal substrate that will allow for a depth of more than 1/10 of a mil of ceramic on each side of the cone and thereby reduce the representative amount of metal in the cone.